1. Field of the Invention
The present invention generally relates to color toners used in electrophotography, and, more particularly, to a color toner that is fixed onto a recording medium by means of the optical energy of flashlight.
2. Description of the Related Art
Electrophotography is a technique widely used in image forming apparatuses such as copying machines, electrophotographic facsimile machines, and electrophotographic printers. As disclosed in U.S. Pat. No. 2,297,691, an electrophotographic technique using photoconductive insulating body has been widely used. According to this technique, an electrostatic latent image is formed by emitting light, such as laser or LED, onto a photoconductive insulating body charged by corona discharges or a charge supply roller. Resin particles called toner that is colored with pigments or dyes are electrostatically fixed to the electrostatic latent image to obtain a toner image. This toner image is then transferred onto a recording medium such as paper or film. At this point, the toner image is only placed on the recording medium, and, therefore, needs to be fixed to the recording medium. Accordingly, the toner is melted on the recording medium by heat, pressure, or light, and are then solidified. Thus, a toner image fixed onto the recording medium can be obtained.
As described above, the toner fixing is carried out by melting toner mainly made up of thermoplastic resin (hereinafter referred to as "binder resin") and then fixing the melted toner onto the recording medium. Well-known examples of such toner fixing methods include a heat-roll technique in which a recording medium having a toner image formed thereon is heated and pressurized directly by a roller, and a flash fixing technique in which toner is fixed onto a recording medium by flashlight from a xenon flash lamp, for instance.
According to the flash fixing technique, toner is melted and fixed onto a recording medium by converting light energy originated from flashlight of a discharge tube of a xenon flash lamp into thermal energy.
This flash fixing technique has advantages over the heat-roll technique in an image forming apparatus. The advantages include: (1) the resolution of the toner image formed on a photoconductive insulating film does not deteriorate by the non-contact fixing; (2) no warm-up time is necessary for the image forming apparatus, and an image forming operation can be promptly started; and (3) the fixing can be carried out on recording media of any material or thickness, such as adhesive paper, pre-printed paper, and paper having various thicknesses.
The process of fixing toner onto a recording medium by flashlight are as follows. Flashlight emitted from a discharge tube is absorbed by a toner image (particle image) formed on the recording medium, and is then converted into thermal energy. The toner is heated up accordingly, and is melted. As a result, the toner adheres to the recording medium. After the flashlight emission, the temperature drops, and the melted toner solidifies to form a fixed toner image.
In the spectral distribution of the xenon flash lamp widely used as a discharge tube for flash fixing, the light emission strength is very high in the near-infrared wavelength region of 800 nm to 1100 nm, and is relatively low in the visible region of 400 nm to 800 nm. Accordingly, toner for flash fixing needs to have high light absorptivity with respect to light in the near-infrared wavelength region.
However, binder resin that is the main component of toner generally has very low light absorptivity in the visible region and the near-infrared region.
When the colorant is black, toner has high light absorptivity in the visible region and the near-infrared region. When the colorant is a coloring material, such as yellow, cyan, magenta, red, blue, or green, toner has acceptable light absorptivity in the visible region, but has only low light absorptivity in the near-infrared region. As a result, it is difficult to fix the color toner containing the binder resin and the coloring material by flashlight only strong enough to fix black toner. Accordingly, the fixing of the color toner requires strong light energy.
To fix color toner onto a recording medium by flashlight, there have been techniques of adding infrared ray absorption agent in the near-infrared wavelength region, which is the light emission wavelength region of a xenon flash lamp, thereby reducing the use of light energy. For instance, Japanese Laid-Open Patent Application Nos. 61-132959, 6-118694, and 7-191492 each disclose a technique in which an aminium compound or a imonium compound is added to toner for flash fixing. In Japanese Laid-Open Patent Application No. 6-238056, resin particles including infrared ray absorption agent containing anthraquinone dye, polymethine dye, or cyanine dye, are attached to the surfaces of toner particles. Also, in Japanese Laid-Open Patent Application No. 10-39535, tin oxide or indium oxide is added to toner so as to increase the fixity of color toner by flashlight.
In the above disclosed techniques, infrared ray absorption agent is added to color toner, so as to promote the conversion efficiency of converting light energy into thermal energy, and to increase the meltability of the binder resin.
However, the addition of the infrared ray absorption agent is not enough the melt the binder resin. Also, the aminium compound and the imonium compound used as the preferred infrared ray absorption agent is colored, and a large amount of those compounds will have adverse influence on a fixed color image. Accordingly, it is preferable to use a small amount of infrared ray absorption agent.
As described so far, a greater amount of light energy is required for fixing color toner by flashlight.